The invention relates to a color television display device comprising a color picture display tube and a deflection coil unit provided with a deflection coil for deflecting into the vertical direction, the electron beams generated in the display tube, the coil being divided into two series-interconnected coil halves and being connected to a deflection current generator for generating a sawtooth deflection current flowing in operation through the deflection coil halves and having a trace and a retrace, which reverses its direction at approximately the center instant of the trace period and further provided with an adjustable balancing resistor in a parallel branch which produces a correction current which flows through the deflection coil halves.
Such a device is disclosed in U.S. Pat. No. 2,825,846. In order to eliminate asymmetry between the magnetic deflection fields generated by the coil halves, the coil halves in this known device are preferably series-connected, so that the deflection currents flowing therethrough are equal in principle, and each coil half is shunted by an adjustable resistor. By adjusting this resistor the current through the associated coil half can be adjusted individually, namely so that the resultant vertical deflection field is symmetrical with respect to the electron beams. If such a correction were not used convergence errors into the vertical direction would be the result. In the center of the display screen of the picture display tube where the deflection currents are substantially zero, there is a proper registration of colored horizontal lines. However, at the top and at the bottom of the screen, such lines would not be in registration. The asymmetry thus corrected can be caused by an irregularity in the construction of the tube, particularly in the electron gun(s) thereof, and/or in the deflection coil unit, by a deviation in the correct position of this unit, etc.
The asymmetry can also be corrected in a different manner. Namely, the correction described above may be considered as a correction by means of a so-called differential current, this being a sawtooth current which is subtracted in one coil half from the deflection current flowing therethrough, whereas in the other coil half it is added to the deflection current flowing therethrough, these two deflection currents being equal. As known, a differential current generates a so-called four-pole field which is superimposed on the deflection fields. Devices are known wherein the correction four-pole field is generated in a manner different from the manner described in the above-mentioned U.S. patent, but they are of a more intricate construction and more difficult to adjust. They require additional windings around the neck of the display tube or on the core of the deflection coil unit, which windings are passed through by currents which are generated by special generators. Consequently preference should be given to the described, simple correction method.
However, it appeared that the presence of the balancing resistors may have an adverse effect on the convergence, so that, for example, a vertical convergence error is visible in the center of the display screen while the place where no error can be observed is not located in the center but somewhat higher. This does not only indicate an error in the static convergence but also an asymmetry in the operation of the resistors between the top and the bottom of the displayed picture. The static convergence error can be compensated for by means of static correction means but this requires a laborious and time-consuming adjustment of the static convergence. Furthermore, there are color picture display tubes wherein such correction means are completely missing.